Of the many conversion processes known in the refining art, hydrocracking has assumed a greater importance over the years since it offers the refiner product flexibility combined with product quality.
There has been considerable effort devoted to the development of hydrocracking catalysts which combine high cracking activity with a low tendency to overcrack towards light products and, in particular, to the less valuable C.sub.1 -C.sub.3 and C.sub.4 gaseous by-products.
Kerosine or gas oil (middle distillates) are often the desired products of a hydrocracking process. However, hydrocracking catalysts with a high selectivity towards middle distillates tend to have a low cracking activity. Such catalysts are typically based on a single, active cracking component such as an aluminosilicate, especially a Y zeolite component.
It is known, e.g. from International (PCT) Patent Specifications Nos. WO 91/17829, WO 92/09366 and WO 97/20016, to form composites of two different cracking components, e.g. Y zeolite and zeolite beta aluminosilicates, for use in hydrocracking, or other hydro or chemical conversion processes. Such composite catalysts are commonly prepared by the conventional techniques of combining the active cracking component(s) with a binder or binder-forming material and then extruding the mixture, drying the extrudates so-formed and calcining. Specifically the combination of materials is carried out by co-mulling the ingredients together in mix-muller equipment, usually with water and a peptising agent.